The present invention relates generally to semiconductor laser dies and, more particularly, to a die having a vertical cavity surface emitting laser (xe2x80x9cVCSELxe2x80x9d) and providing a balanced output to a drive circuit for optimally driving a VCSEL.
Semiconductor lasers are employed in numerous applications such as pumping solid state lasers, forming laser arrays, serving as sources for optical pick-up in compact disc (CD) players, and coupling to optical fibers in optical communications applications. Traditionally, the most common form of semiconductor laser has been the side or edge-emitting laser, though more recently VCSELs have been used in the above applications and indeed have become the dominant laser source in numerous data communications applications.
In contrast to the edge-emitting laser, in which the active region is positioned within a resonance cavity defined by two reflective layers positioned at opposing sides of the active region, VCSELs, in one form, have a resonance cavity defined by two reflective layers positioned at the top and bottom of the active region to produce a vertical emission, i.e., an emission normal to the junction plane of the active region. The junction plane may be a plane defined by the intersection of an AlGaAs layer and a GaAs layer in a multi-quantum well VCSEL structure, for example. In sum, edge-emitting lasers have a resonance cavity parallel to the junction plane and an emission through the side of the laser, while VCSELs have a resonance cavity orthogonal to the junction plane and emit through a surface of the laser.
Biasing a VCSEL is achieved through contact layers in the form of thin metal layers, where the contact layers, being photo-opaque (i.e., photon absorbing, over the emission spectrum of VCSELs), are positioned at specific locations on the top and bottom of a layered semiconductor structure surrounding the active region. For example, designs include a first metal contact layer over the entire bottom surface of the semiconductor substrate (i.e., between the semiconductor laser and a mounting substrate) and a second metal contact layer that is disposed either over half of the upper surface of the semiconductor substrate, only at a corner of the upper surface, or over all but an emission window of the top surface of the semiconductor laser. Still other designs use the bottom surface of the semiconductor laser as an emission surface with the mounting substrate upon which the VCSEL is mounted being photo-transparent.
In another form known in the art, a vertically-emitting laser is created from an edge-emitting semiconductor laser with an external cleaved surface angled at 45 degrees to the junction plane in the active region. Using materials with a high reflectivity over the principal wavelength(s) emitting from the lasing region, the cleaved surface reflects vertically the horizontally emitted light from the edge of the lasing region. This forms a surface-emitting laser, but without a vertical cavity. More recently, tunable VCSELs that use MEMs (micro-electro-mechanical-structures) to mechanically move the upper reflective region with respect to the active region have been shown. By lengthening or contracting the resonance cavity, one can tune the resonating wavelength of the coherent photonic emission for such VCSELs.
VCSELs have become the dominant laser source in demanding optical data communications systems like the Gigabit Ethernet standard provided for in the IEEE 802.3z protocol and the Fibre Channel standard provided in the ANSI X3.T11 protocol. VCSELs are preferred because they have high modulation bandwidths and can produce high bit transmission rates.
The Gigabit Ethernet standard is designed to improve upon the Ethernet (10 Mbps) and Fast Ethernet (100 Mbps) standards by providing a way to transmit and receive large amounts of data at data rates of 1 Gbps. The Gigabit Ethernet standard is intended for use in such demanding applications as scientific modeling, data warehousing, data mining, internet/extranet access, backing-up networks, and high-quality video conferencing. The Gigabit Ethernet standard achieves higher bandwidth while maintaining the simplicity and the relatively low cost of implementation and maintenance associated with the now-entrenched Ethernet standard. Providing higher bandwidth using low-cost components has made the Gigabit Ethernet standard attractive. With a large percentage of the installed network connections being Ethernet based, the Gigabit Ethernet standard has the advantage of backward compatibility with existing Ethernet backbones, as well. For example, all three Ethernet standards (Ethernet, Fast Ethernet, and Gigabit Ethernet) use the same IEEE 802.3 frame format. An additional feature of the IEEE 802.3z protocol is that it allows for full and half-duplex operation.
Fibre Channel, the ANSI X3.T11 protocol, is used in data storage and access systems in lieu of Small Computer System Interface (SCSI) systems. SCSI systems use an individual SCSI controller for each storage device (e.g., a hard drive) connected to a network. The separate parallel connections that result consume space and, as more storage devices are connected, decrease the I/O processing efficiency of the system, with SCSI applications typically having throughput speeds of less than 100 Mbps. The Fibre Channel standard allows serial I/O connection of numerous devices to a single input of a data processor or server and can achieve throughput in excess of 1 Gbps, and the serial nature of the Fibre Channel standard allows hot-plug connection of storage devices xe2x80x9con the flyxe2x80x9d, e.g., without taking the system offline. The Fibre Channel standard also allows access to devices many meters from the processor or server because of the use of optical fiber in place of the copper cable used in SCSI applications.
VCSELs, serving as optical signal sources for these and other optical fiber-based data communication applications, are preferred over edge-emitting lasers for numerous reasons, one reason being the beam shape of the output. The beam shape of the output of the edge-emitting lasers looks approximately like the cross-sectional shape of the active region: the beam shape is elliptical. In contrast, the beam shape of the output of a VCSEL is approximately circular, matching the circular shape of the emission window defined by the upper contact layer and the active region, which produces a uniform photon emission across this window. The output of a VCSEL also has a low numerical aperture. Both of these properties make fiber coupling (in particular single-mode fiber coupling) easier with a VCSEL than with an edge-emitting laser. With a circular beam shape, the VCSEL output can be focused into a single mode optical fiber, using a known ball lens, for example, thus reducing undesirable multimode optical-fiber losses such as intermodal dispersion.
Furthermore, VCSELs are characterized by high power conversion efficiency, even in low input power ranges, and provide wide small-signal modulation bandwidths, with modulation bandwidths in excess of 1 GHz. Both of these advantages demonstrate the ability of VCSELs to be used in Gigabit Ethernet and Fibre Channel applications at a relatively low drive current and, thus, with less power usage and less thermal loss.
Moreover with the vertical emission, VCSELs can be relatively easily packaged, often with a photodetector disposed at the back surface of the VCSEL (or within the substrate upon which the VCSEL is mounted) for power monitoring and feedback control. More recently, VCSELs have been packaged in a transceiver for use in duplex communication.
VCSEL active region layers are deposited by known techniques and are doped to form PN junctions, PIN junctions with an intrinsic layer disposed between the p-type and n-type layers, or double heterostructures. Furthermore, VCSELs have relatively fast transition times, e.g., the VCSEL output can be driven from a binary xe2x80x9c0xe2x80x9d state to a binary xe2x80x9c1xe2x80x9d state in a relatively short period of time. Therefore, VCSELs are able to achieve wide modulation bandwidths, with transmission rates of between 1 and 10 Gbps achievable. Moreover, VCSELs have been designed to operate at wavelengths desirable for optical fiber transmission, such as 850 nm with multimode optical-fibers and 1270-1600 nm with single mode optical-fibers.
Though VCSELs are commonly used, the demands for increased bandwidth and better signal integrity in gigabit data communication environments like Gigabit Ethernet and Fibre Channel suggest a need for more efficient, cleaner, and non-interfering VCSEL operation. Part of these demands result from the increased functionality of the gigabit communication environment, where, for example, the challenge of making smaller components, particularly smaller PCBs (printed circuit boards) in optical transceivers, has resulted in smaller, more confined opto-electronic components. These smaller configurations, however, increase the potential for crosstalk between fibers. Furthermore, electromagnetic interference (EMI) is also increased due to the short distances between the closely spaced opto-electronic drive circuit components.
In addition to these considerations, current drive circuits do not provide clean input signals to the VCSEL at high frequencies. Analyzing the electrical characteristics of a VCSEL shows that the impedance of the VCSEL, which is essentially resistive at lower drive frequencies, varies greatly at higher drive frequencies, especially above 1 GHz, i.e., the driving frequencies in Gigabit Ethernet and Fibre Channel applications. In fact, when analyzing the impedance of a VCSEL die chip and the accompanying packaging or housing of the VCSEL, for example a known TO packaging arrangement, the electrical parasitic effects from the package and from the die result in an even more pronounced frequency dependence of the impedance seen by the drive circuit. That is, VCSEL packaging adds to the non-linearity of VCSEL operation at high frequencies. In short, as an active circuit element, the VCSEL has an impedance that varies substantially with frequency and which in turn results in a non-linear load for the VCSEL drive circuit, particularly over high frequency operation.
To achieve relatively low drive currents, and thereby provide relatively low power demands for use in Ethernet switches, routers, hubs, end-user connections, etc., drive circuits for VCSELs, or any semiconductor laser for that matter, typically have a differential amplifier configuration that includes at least two transistors in a common emitter or common collector configuration. In the Ethernet or Fibre Channel environment, differential signaling is commonly used to reduce EMI and maintain electrical signal integrity. This design philosophy is continued inside the laser drive, where the input digital differential signal is converted to an analog laser drive current.
The cleanest electrical signal is typically obtained with a pair of transistors in a differential mode, operating either as common collector or common emitter.
The VCSEL is connected to the drive circuit in one of two driving modes. First, the VCSEL may be operated in a single-ended mode in which the laser is connected between the output of one of the transistors and ground. In this configuration, the output of the second transistor of the differential amplifier pair is matched to the output of the VCSEL through the use of a resistive element connected to the output of the second transistor. The resistive element resides either on the differential amplifier circuit or the PCB and approximates the low frequency impedance of the VCSEL.
Second, the VCSEL laser may be operated in a differential drive mode in which the VCSEL laser is connected between the output of both transistors. This mode has the advantage of lower voltage swings in response to abrupt changes in the signal current. Compared to the single-ended drive mode, the differential drive mode produces faster rise times, e.g., transitions from a xe2x80x9c0xe2x80x9d state to a xe2x80x9c1xe2x80x9d state, with less overshoot. However, differentially driving the VCSEL can be problematic because of the potential asymmetry introduced by injecting carriers into both the N and P side of the junction. As the speed of operation increases, this asymmetry leads to reduced optical performance.
The single-ended drive mode suffers from an impedance-matching problem resulting from the reactive component of the impedance of the VCSEL. With the impedance seen by the differential amplifier depending on numerous factors, e.g., shape of the VCSEL, drive current, shape and size of wire bond pads and wire leads, it is difficult to ensure that the optimum waveform is provided to the VCSEL or that the output from the VCSEL will be at the highest bit rate possible.
This difficulty has presented a problem to manufacturers who have, thus far, used resistors as xe2x80x9cmatchingxe2x80x9d loads in single ended drives. Resistors provide a relatively acceptable matching of the low-frequency impedance of the VCSEL and, being non-reactive circuit elements, do not contribute to the electrical parasitics in the drive circuit. A number of problems result from using these resistors, however.
One problem is that, though VCSELs are batch manufactured from cutting out individual lasers from a semiconductor wafer, individual VCSELs vary in impedance characteristics, both within a wafer and in wafers manufactured at different times. Matching a load in the differential amplifier circuit with the load impedance of a VCSEL requires individual testing of the impedance characteristics of the particular VCSEL being driven, a time-consuming and expensive endeavor.
Another problem exists in that VCSELs have a frequency dependent impedance. The reactive component of the VCSEL impedance varies greatly with the frequency of the driving current, such that at high frequency, e.g., GHz frequencies, the mismatch between the impedance of the VCSEL and the resistor impedance is substantial. For example, the inductance of a VCSEL is on the order of 1 nH, and the capacitance is on the order of 1 pF. The current injection efficiency into this load at speeds varying from 1-10 GHz can be decreased by more than 50%, resulting in poor optical performance. Distortion of the waveform to the VCSEL results from the mismatch, as the output waveform may show overshoot on the rising edge of the output (the xe2x80x9c0xe2x80x9d to xe2x80x9c1xe2x80x9d state transition) and a slower trailing-off of the falling edge (the xe2x80x9c1xe2x80x9d to xe2x80x9c0xe2x80x9d state transition) when compared to the waveform input to the drive circuit.
Therefore, it is desirable to have a more closely matched load for use with differential drive circuitry used in VCSELs, or any semiconductor laser, and it is desirable to create such a more closely matched load in a convenient and economical manner.
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention, and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
In accordance with an aspect of the present invention, a die having a semiconductor laser disposed thereon and having a balancing load disposed thereon is provided. The balancing load has an impedance, including both resistive and reactive components, that is matched to a load impedance of the semiconductor laser.
In some embodiments, the semiconductor laser is a VCSEL, and in some of such embodiments the balancing load is a VCSEL, whereby the output of the balancing load VCSEL is blocked so as not to interfere with a signal output from the semiconductor laser VCSEL.
In some embodiments, the die has a plurality of semiconductor lasers and a plurality of balancing loads, where for each semiconductor laser there is at least one balancing load having an impedance, including both resistive and reactive components, matched to the load impedance of that semiconductor laser.
In some embodiments, the impedance of the balancing load is substantially identical to the impedance of the semiconductor laser for frequencies on the order of about 1 GHz.
In some embodiments, the die is used in a laser package, which includes a header base and a housing having a housing window.
In accordance with another aspect of the present invention, an apparatus comprising a drive circuit, a VCSEL coupled to the drive circuit, and a balancing load coupled to the drive circuit is provided. The balancing load has an impedance matched to an impedance of the VCSEL at frequencies on the order of about 1 GHz.
In some embodiments, the impedance of the VCSEL is substantially identical to the impedance of the balancing load at frequencies on the order of about 1 GHz.
In some embodiments, the drive circuit comprises a differential amplifier, where a first output stage comprises a first transistor connected to the VCSEL and a second output stage comprises a second transistor connected to the balancing load. In further embodiments, the first output stage includes a first reactive circuit block connected to the first transistor and the second output stage includes a second reactive circuit block connected to the second transistor. In even further embodiments, the first reactive circuit block and the second reactive circuit block have substantially the same impedance at frequencies on the order of about 1 GHz.
In some embodiments, the apparatus has a plurality of VCSELs coupled to the drive circuit and a plurality of balancing loads coupled to the drive circuit, forming a laser array where for each VCSEL there is at least one balancing load having an impedance, including both resistive and reactive components, matched to the load impedance of that VCSEL.
In accordance with another aspect of the present invention, a method of using a first VCSEL with a balanced load drive circuit having a first differential amplifier and a second differential amplifier is provided. The method includes (1) forming the first VCSEL on a die substrate, (2) forming a second VCSEL of substantially identical shape and size as the first VCSEL on the die substrate, (3) masking the emission window of the second VCSEL, and (4) connecting the drive circuit to the first VCSEL via the first differential amplifier and connecting the drive circuit to the second VCSEL via the second differential amplifier, so that a selectively balanced load is present at the output stages of the amplifiers.
In some embodiments the forming of the second VCSEL further comprises forming the second VCSEL so that an impedance of the second VCSEL is substantially identical to an impedance of said first VCSEL at frequencies on the order of about 1 GHz.
In accordance with another aspect of the present invention, a method of forming a laser array having a plurality of VCSELs disposed thereon is provided. The method includes (1) selectively forming half of the plurality of VCSELs to each have an emission window exposed for vertical output; (2) selectively forming the other half of the plurality VCSELs such that the outputs from these VCSELs are masked; and (3) pairing each one of the first half of the plurality VCSELs to a corresponding one of each one of the second half of the plurality of VCSELs to form first and second VCSEL pairs and connecting a drive circuit to each pair so that the drive circuit sees a balanced load when driving the corresponding VCSEL pair.
The novel features of the present invention will become apparent to those of skill in the art upon examination of the following detailed description of the invention or can be learned by practice of the present invention. It should be understood, however, that the detailed description of the invention and the specific examples presented, while indicating certain embodiments of the present invention, are provided for illustration purposes only because various changes and modifications within the scope of the invention will become apparent to those of skill in the art from the detailed description of the invention and claims that follow.